As the amount of data for transmission increases, long-distance high-capacity optical transmission using optical amplification technology is being considered. When wavelength-division multiplexing (WDM) is used for high-capacity optical transmission, non-linear effects in the optical fiber transmission line contribute to degradation in transmission characteristics. The non-linear effect of an optical fiber is proportional to n2/Aeff where n2 is the non-linear refractive index and Aeff is the effective core area. Therefore, the non-linear effect of an optical fiber can be decreased by increasing its effective core area.
In the past, the effective core area has been increased by optimizing the refractive index profile of the optical fiber. For example, in U.S. Pat. No. 6,466,721, optical fibers having effective core areas of approximately 80˜120 μm2 at 1550 nm have been disclosed.
However, if the effective core area is increased by optimizing the refractive index profile of the optical fiber, a “confinement” effect of the light transmitted in the optical fiber reduces in the core, and tends to degrade its bending-loss characteristic. Therefore, feasible effective core areas are limited to ranges which can preserve allowable bending-loss characteristics. For example, an allowable bending-loss characteristic can be 10 dB/m or less at a bending diameter of 20 mm to sustain cable manufacturing. Furthermore, if the confinement effect in the core is decreased due to the increase in effective core area, then the microbending loss is also increased. To overcome these issues, M. Tsukitani et al. discloses an optical fiber having a larger outer diameter, which achieves an effective core area of approximately 200 μm2 at 1550 nm, in “Ultra low nonlinearity fiber with improved microbending performance”—OECC2002 Technical Digest, 11D1-3. However, optical fibers in U.S. Pat. No. 6,466,721 do not have large enough effective core areas for long-distance, high-capacity optical transmission required in recent years. In addition, even though M. Tsukitani et al. discloses an optical fiber with an effective core area of approximately 200 μm2 at 1550 nm, either the bending loss at a bending diameter of 20 mm is extremely large (290 dB/m at 1550 nm) or the cut-off wavelength is shifted to a longer wavelength (i.e., 2000 nm). Furthermore, if the cut-off wavelength is shifted to 2000 nm, then it is impossible to have single-mode transmission at 1550 nm where transmission loss is the minimum for silica optical fibers.